Localized Water Reverberation Phases and its Impact on Back-Projection Images

Coherent radiators imaged by back-projections (BP) are commonly interpreted as
part of the rupture process. Nevertheless, artifacts introduced by structure related
phases are rarely discriminated from the rupture process. In this study, we use a
calibration event to discriminate between rupture and structure effects. We re-examine
the waveforms and BP images of the 2012 Mw 7.2 Indian Ocean earthquake and a calibration
event (Mw 6.2). The P wave codas of both events present similar shape with characteristic
period of approximately 10 s, which are back-projected as coherent radiators near
the trench. S wave BP doesn’t image energy radiation near the trench. We interpret
those coda waves as localized water reverberation phases excited near the trench.
We perform a 2D waveform modeling using realistic bathymetry model, and find that
the steep near-trench bathymetry traps the acoustic water waves forming localized
reverberation phases. These waves can be imaged as coherent near-trench radiators
with similar features as that in the observations. We present a set of methodologies
to discriminate between the rupture and propagation effects in BP images, which can
serve as a criterion of subevent identification.